When your server is located in a data center far away, it makes it difficult to walk over to the box and read the service tag off of it. Fortunately, the Service Tag is stored somewhere in the system BIOS, and is accessible with the ‘dmidecode’ utility.
[root@host ~]# dmidecode|grep "Serial Number" Serial Number: 80NZV71
You’ll probably see several other serial numbers in there as well for things like your hard drives or other devices. The Dell shouldn’t be too difficult to pick out. I think they are always 7 digits and have letters in middle. There is lots of other interesting things in the ‘dmidecode’ output too, like the speed and type of each RAM module installed, and a description of all of the onboard devices (ie: video and network cards)
Thanks to @kleinbaas who commented below how to do the same thing on a Windows machine:
C:\Documents and Settings\brandon>wmic bios get serialnumber SerialNumber GX245D1